1. Field of the Invention
This invention relates to metal fabrication techniques and in particular to methods for optimizing manufacturing variables as a function of material properties.
2. Description of the Prior Art
One of the most important challenges facing the aerospace industry is cost reduction of structures in today's high performance aircraft. Improvements in the use of expensive raw materials, such as titanium alloys, and in the efficiency of the processes required for fabrication of airframe structures provide a major avenue for reduction of system acquisition costs.
In recent years, attention has been focsued on reducing these costs by developing manufacturing processes that improve material utilization factors and that can produce net or near-net parts. Superplastic forming (SPF) and superplastic forming combined with diffusion bonding (SPF/DB) are two such processes which offer substantial cost savings by reducing the number of detail parts and mechanical fasteners, by achieving more efficient load carrying structures, and by increasing the fly-to-buy ratio of titanium alloys. While these two processes offer substantial savings, their full potential has not yet been realized, one reason being the lack of guidelines to aid the designer in achieving producible configurations with acceptable finished properties.
The superplastic forming/diffusion bonding (SPF/DB) process simultaneously utilizes two phenomena occurring in titanium alloys. One phenomenon, called superplasticity, is the ability of a metal to elongate extensively under low applied stresses at elevated temperatures without localized thinning. The other phenomenon is solid-state diffusion bonding, for which titanium is particularly suited. With this ability to severely form and readily bond, the need to machine, cut, form, drill and assemble small details is virtually eliminated and thus lightweight, monolithic aircraft structures can be produced at low cost.
During SPF and SPF/DB, sheet alloys are exposed to high temperatures for long periods of time. This exposure causes microstructural changes, including grain growth and redistribution of the alpha-beta phases. These changes during SPF/DB are undesirable because they can degrade the finished part properties, decrease high temperature ductility, and increase the flow stress required for superplastic deformation. The large grain size resulting from SPF degrades many room temperature mechanical properties. Decreased high temperature ductility can increase forming time and/or pressure or cause failure of parts during forming.
The grain growth occurring during the superplastic forming causes the flow stress required for further deformation to increase, thus resulting into microstructural hardening. Until recently, it was assumed that the microstructure of titanium alloys did not change during the superplastic deformation and thus the microstructural hardening was neglected. Most of the data available in the literature on this subject relate the flow stress (.sigma.) to the strain rate (.epsilon.) using the following equation: EQU .sigma.=K.epsilon..sup.m ( 1)
where K and m are constants dependent on test parameters, such as temperature and strain rate. m is called the strain rate sensitivity exponent and has considerable significance in determining the stability of flow.
This widely used relationship, however, does not take into account the microstructural hardening resulting from the concurrent grain growth. Thus, the pressure profile predicted using the relationship of Equation (1) will result in underforming or fracture of the part.
All these problems tend to increase in-process scrap and manufacturing costs. In order to reduce the in-process scrap and to get good quality finished parts consistently and without significant degradation of the finished properties, the microstructural hardening which results from the microstructural changes occurring during forming should be taken into account. However, such design considerations can be followed only if the microstructural changes occurring during superplastic forming can be predicted under various forming conditions.
Superplasticity is a phenomenon of very large (200 percent or greater) neck-free extensions of alloy specimens in tensile deformation at elevated temperatures and within a small range of strain rates. Superplasticity can be induced both in materials possessing a stable, ultra-fine grain size at the temperature of deformation greater than 0.4T.sub.m (T.sub.m is the absolute melting temperature) and in those subjected to special environmental conditions, such as thermal cycling through a phase change. These two categories are best described as "structural" and "environmental" superplasticity, respectively. The present invention is concerned with the "structural" type of superplasticity.
Various hypotheses and theoretical models have been developed to account for the superplasticity phenomenon. One of the earliest interpreted the behavior of an elongated alloy in terms of the then popular "amorphous cement" theory of inter-crystalline cohesion, wherein it was suggested that rolling partially converted the metal into an amorphous condition and the elongation was the result of flow in the amorphous regions. Others have attributed the effect to the presence of a very fine grain size, a condition now known to be necessary for the occurrence of structural superplasticity.
Over the past 20 years or so, considerable interest and research activity have been directed to investigating the superplasticity phenomenon. As the number of alloys exhibiting structursal superplasticity has increased, earlier theories have been found inadequate. The more recently developed theories are based on either considerations that take into account the actual atomic structure of the metallic materials or rheologcy (continuum treatments with or without the incorporation of microstructural details like grain size). These various theories and models, however, are generally found to be deficient, for one reason or another, in providing equations and operating hypotheses of sufficient validity and reliability to enable one to use them in a production environment.
Without going into the details of the contemporary atomistic theory relating to superplasticity, a few of the applicable considerations may be noted. The essential requirements for the occurrence of superplasticity include deformation temperature greater than 0.4T.sub.m (T.sub.m is the melting temperature), so that diffusional mechanisms are dominant; fine equiaxed grain structure with grain size less than 10 to 15 microns, so flow stresses are low and the grain boundary sliding contribution to strain is large; and a two-phase microstructure to retard grain coarsening.
Macroscopically, the reason for the extensive elongations and the stability of the flow is that the strain rate sensitivity of the flow stress (m=.differential. ln .sigma./.differential. ln .epsilon.) is high (greater than 0.4) at its deformation temperature and within a certain range of strain rates. Several experiments utilizing optical and scanning electron microscopy indicate that grain boundary sliding is an important deformation mode. The fact that voids do not form in the bulk of the material (the density does not change appreciably during deformation) shows that grains change their shape to accommodate the grain boundary sliding.
The principal mechanical properties of superplastic alloys can be considered as divided into three stages. At low (1.times.10.sup.-6 to 1.5.times.10.sup.-5) and high (1.5.times.10.sup.-3 to 0.1) values of strain rate, strain rate sensitivity (m) is low, while at intermediate values (approximately 1.times.10.sup.-4 to 1.times.10.sup.-3), m is high, which produces superplastic behavior. In the third stage (high values of strain rate) behavior of the superplastic alloys is similar to conventional materials deformed at high temperature. The intermediate stages are characterized by high values of the strain rate sensitivity parameters, whereas in the first stage deformation occurs at extremely low strain rates. Deformation at low strain rates causes longer high temperature exposure of the alloys, which results in substantial grain growth.
Different atomistic models have been proposed by investigators to explain the behavior of superplastic materials. These models have led to defining relations between stress, strain, strain rate, temperature, grain size and other atomistic parameters of the material. Most such models apply to only one of three stages referred to above. Some investigators have tried to explain the stress-strain rate behavior over the three stages by combining different single-stage models.
In the third stage, different dislocation creep models have been employed to explain the superplastic behavior. In the second regime, all experimental observations show that grain boundary sliding plays an important role during deformation. To prevent formation of cavities at triple junctions, grain boundary sliding may be accomplished by controlling mechanisms, such as boundary migration, lattice or boundary diffusion, and dislocation creep in neighboring grains.
As the phenomeneon of superplasticity moves from the laboratory into the production arena, the need for quality and process control during superplastic forming becomes increasingly important. This calls for laboratory characterization of superplastic metals which can reflect their behavior in actual forming applications. Such requirements are not limited to determining tensile elongations under optimum superplastic condition, but include detailed characterization of constitutive relations for superplastic flow, since a reasonably accurate control of pressurization cycle during forming is required. Real problems arise in this area because of the complexity associated with microstructural changes occurring simultaneously during forming.
Many investigators have tried to define the relationship between the flow stress and the strain rate during the elevated temperature deformation, and this relationship has been described through Equation (1) hereinabove. In working with that equation, the flow stress versus strain rate data are obtained from step strain rate tests in which the tensile specimen is subjected to varying pulling speeds and the stress at which the load becomes level at each speed is plotted against the strain rate for that speed in the form of a log-log curve of the measured variables. Quantitative values of strain rate sensitivity are then determined from the slope of the curve.
Even though this technique has been used by several investigators to characterize the superplastic flow behavior, there are some major deficiencies in this approach such as:
(a) The step strain rate test is used only up to small strain levels. The information obtained from a relativey short test ignores the microstructural changes occurring during the superplastic deformation over large strains.
(b) For a constant strain value for each strain rate, the time at temperature is longer for slow strain rates and shorter for fast strain rates. This causes differences in structure due to grain growth. Hence the data obtained are not reliable.
(c) Because the grain growth occurs as a function of time at temperature, the microstructure in the early stages of the test is much different than that in the final stages of the test. This will give additional increase in the flow stress. This grain-growth induced hardening, however, is not corrected for by this technique.
The method of the present invention relates particular material properties and manufacturing variables to the forming characteristics and physical properties of the specific items being formed by SPF and SPF/DB in a manner which optimizes the forming process.